1. Field of the Invention
The invention relates to an MR method for determining the nuclear magnetization distribution in an examination zone, in which the image data acquired during an MR examination by means of a surface coil arrangement, having a locally inhomogeneous sensitivity and comprising at least one coil, are corrected on the basis of auxiliary values derived from the data acquired by means of a body coil arrangement having a locally at least approximately homogeneous sensitivity.
2. Description of the Related Art
The determination of the nuclear magnetization distribution in an examination zone by means of a surface coil arrangement comprising at least one surface coil, but in practice usually several surface coils, offers the advantage that the signal-to-noise ratio is substantially better than in the case of coils capable of receiving the spin resonance signals from the entire examination zone; the latter coils, also referred to as whole-body coils or volume coils in practice, will be referred to as a body coil arrangement hereinafter.
The formation of MR images by means of a surface coil arrangement, however, has a drawback in that the sensitivity (the location-dependent sensitivity of an MR coil is defined as the quotient of the magnetic induction at a given location and the current in the coil causing this magnetic induction at this location) is locally inhomogeneous, i.e. dependent on the location where the MR signal occurs in the examination zone. Therefore, the data produced by means of a surface coil arrangement must be corrected so as to eliminate the location-dependent sensitivity differences.
To this end it is known to use a body coil arrangement in addition. Because such an arrangement has a locally homogenous sensitivity, the MR image produced by means of the body coil can be used to correct the data for the MR image formed from the MR signals received from the surface coil arrangement.
In a first method of this kind which is known from EP-A 412 824 as well as from an article by Okamoto et al., SMRM 11th Annual Meeting, p. 4042, 1992, auxiliary values for the correction of the location-dependent sensitivity of the surface coil arrangement are derived from the MR signals acquired by the body coil arrangement simultaneously with the surface coil arrangement.
For this method to be effective it is necessary that no inductive coupling exists between the individual coils of the surface coil arrangement on the one side and the body coil arrangement on the other side. In practice this condition can be satisfied for special cases at best.
In another method, disclosed in EP 271 123, which corresponds to U.S. Pat. No. 4,812,753 the MR examination is carried out separately in time by means of a surface coil arrangement, comprising a single coil, and as an MR auxiliary measurement (even though it also concerns an MR examination, it will be referred to hereinafter as "MR auxiliary measurement") by means of the body coil arrangement. Even though the MR auxiliary measurement can be carried out with a spatial resolution which is less than that of the MR examination by means of the surface coil arrangement, the two MR methods must produce the same contrast behaviour. Consequently, the duration of the MR auxiliary measurement by means of the body coil arrangement is comparatively long and depending on the acquisition method. Moreover, small-volume changes which succeed one another in rapid succession in the zone covered by the two coil arrangements, for example as they may occur in MR angiograms, are a cause of failure of the method, because even the smallest spatil shift between the two images lead to errors.
It is an objective of the present invention to improve a method of the kind set forth. This objective is achieved in accordance with the invention in that, in addition to the MR examination a respective MR auxiliary measurement is carried out by means of the surface coil arrangement as well as by means of the body coil arrangement, in that the two MR auxiliary measurements are carried out temporally separate from one another and from the MR examination, in that during the two MR auxiliary measurements the magnetic fields acting on the examination zone exhibit the same temporal variation, and that the auxiliary values are derived from the data acquired during the two MR auxiliary measurements.
In addition to the MR examination by means of the surface coil arrangement, therefore in accordance with the invention there a (first) MR auxiliary measurement by means of the surface coil arrangement and a (second) MR auxiliary measurement by means of the body coil arrangement are carried out, it being essential that the magnetic fields acting on the examination zone during the two MR auxiliary measurements exhibit the same temporal variation, i.e. that the same sequences act on this zone. In comparison with EP-B 271 123, requiring only one MR examination and one MR auxiliary measurement, it appears in first instance that more time is now required. However, because the two MR auxiliary measurements need not produce the same contrast behaviour as the MR examination, very fast MR methods can be used for this purpose. Moreover, the MR auxiliary measurements require a low spatial resolution only, so that the measuring time for the two MR auxiliary measurements is even further reduced. Furthermore, no problems are encountered when a human vascular system is to be imaged by means of the MR examination, because the two MR auxiliary measurements need not image the vascular system (but only the area in which these vessels are situated), so that the positional changes occurring in the vascular system between the two MR auxiliary measurements have no effect whatsoever on the calibration.
In a further embodiment of the invention, the image data acquired during the MR examination are corrected in that MR superposition images are calculated from the MR image data acquired during the MR auxiliary measurement by means of the surface coil arrangement on the one hand and from the MR image data acquired during the MR examination on the other hand, which MR superposition images are dependent on the location-dependent sensitivity of the surface coil arrangement in the same way, and that auxiliary values are derived from the relation of the MR superposition image obtained from the MR auxiliary measurement to the MR image obtained from the MR auxiliary measurement by means of the body coil arrangement, which auxiliary values are used to correct the MR superposition image derived from the MR examination.
In a preferred embodiment which offers an optimum signal-to-noise ratio, the MR superposition images are formed by weighted summing of the MR images determined by the individual coils of the surface coil arrangement, the weighting factors that are applied to the MR image data of the individual MR images to be summed being derived from the relation of the respective MR image data determined for the same pixel during the two MR auxiliary measurements.
An MR device for carrying out the method in accordance with the invention, includes:
a) a surface coil arrangement comprising at least one coil, PA1 b) a body coil arrangement of locally at least approximately homogeneous sensitivity, PA1 c) processing means for forming MR images from the MR signals received from the surface coil arrangement and from the body coil arrangement as well as for deriving auxiliary values from the MR images, and PA1 d) programmable control means for controlling the variation in time of the magnetic fields acting on the examination zone, and for controlling the MR signal processing is constructed so that the control means are programmed in such a manner that an MR examination, an MR auxiliary measurement by means of the surface coil arrangement, and an MR auxiliary measurement by means of the body coil arrangement are carried out temporally separately, and that the processing means derive auxiliary values from the data acquired during the MR auxiliary measurements, said auxiliary data being used to correct the image data acquired during the MR examination.
In a further embodiment of the invention, the surface coil arrangement and the body coil arrangement can operate in a receiving mode in which they can pick up MR signals and in an decoupling mode in which they cannot pick up MR signals, and that the modes are controlled by the control means in such a manner that the body coil arrangement operates in the decoupling mode when the surface coil arrangement operates in the receiving mode, and vice versa.